Eukaryotic cells constantly degrade and replace cellular protein. This permits the cell to selectively and rapidly remove proteins and peptides having abnormal conformations, to exert control over metabolic pathways by adjusting levels of regulatory peptides, and to provide amino acids for energy when necessary, as in starvation. See Goldberg, A. L. & St. John, A. C. Annu. Rev. Biochem. 45:747-803 (1976). The cellular mechanisms of mammals allow for multiple pathways for protein breakdown. Some of these pathways appear to require energy input in the form of adenosine triphosphate ("ATP"). See Goldberg, A. L. & St. John, supra.
Multicatalytic protease (MCP, also typically referred to as "multicatalytic proteinase," "proteasome," "multicatalytic proteinase complex," "multicatalytic endopeptidase complex," "20S proteasome" and "ingensin") is a large molecular weight (700 kD) eukaryotic non-lysosomal proteinase complex which plays a role in at least two cellular pathways for the breakdown of protein to peptides and amino acids. See Orlowski, M. Biochemistry 29(45) 10289-10297 (1990). The complex has at least three different types of hydrolytic activities: (1) a trypsin-like activity wherein peptide bonds are cleaved at the carboxyl side of basic amino acids; (2) a chymotrypsin-like activity wherein peptide bonds are cleaved at the carboxyl side of hydrophobic amino acids; and (3) an activity wherein peptide bonds are cleaved at the carboxyl side of glutamic acid. See Rivett, A. J. J. Biol. Chem. 264: 21 12215-12219 (1989) and Orlowski, supra.
One route of protein hydrolysis which involves MCP also involves the polypeptide "ubiquitin." Hershko, A. & Crechanovh, A. Annu. Rev. Biochem. 51:335-364 (1982). This route, which requires MCP, ATP and ubiquitin, appears responsible for the degradation of highly abnormal proteins, certain short-lived normal proteins and the bulk of proteins in growing fibroblasts and maturing reticuloytes. See Driscoll, J. and Goldberg, A. L. Proc. Nat. Acad. Sci. U.S.A. 86:787-791 (1989). Proteins to be degraded by this pathway are covalently bound to ubiquitin via their lysine amino groups in an ATP-dependent manner. The ubiquitin-conjugated proteins are then degraded to small peptides by an ATP-dependent protease complex by the 26S proteasome, which contains MCP as its proteolytic core. Goldberg, A. L. & Rock, K. L. Nature 357:375-379 (1992).
A second route of protein degradation which requires MCP and ATP, but which does not require ubiquitin, has also been described. See Driscoll, J. & Goldberg, A. L., supra. In this process, MCP hydrolyzes proteins in an ATP-dependent manner. See Goldberg, A. L. & Rock, K. L., supra. This process has been observed in skeletal muscle. See Driscoll & Goldberg, supra. However, it has been suggested that in muscle, MCP functions synergistically with another protease, multipain, thus resulting in an accelerated breakdown of muscle protein. See Goldberg & Rock, supra.
It has been reported that MCP functions by a proteolytic mechanism wherein the active site nucleophile is the hydroxyl group of the N-terminal threonine residue. Thus, MCP is the first known example of a threonine protease. See Seemuller et al., Science (1995) 268 579-582; Goldberg, A. L, Science (1995) 268 522-523.
The relative activities of cellular protein synthetic and degradative pathways determine whether protein is accumulated or lost. The abnormal loss of protein mass is associated with several disease states such as muscular dystrophy, cardiac cachexia, emphysema, leprosy, malnutrition, osteomalacia, child acute leukemia, and cancer cachexia. Loss of muscle mass is also observed in aging, long term hospitalization or long term confinement to bed, and in chronic lower back pain.
With denervation or disuse, skeletal muscles undergo rapid atrophy which leads to a profound decrease in size, protein content and contractile strength. This atrophy is an important component of many neuromuscular diseases in humans. Enhancement of protein breakdown has been implicated as the primary cause of muscle wasting in denervation atrophy. Furono, K. et al. J. Biochem. 265/15:8550-8557 (1990). While the specific process or processes involved in protein hydrolysis in muscle has not been identified, evidence is available linking the involvement of MCP in the accelerated breakdown of muscle proteins. See, for example, Furono, supra, and PCT Published Application WO 92/20804 (publication date: Nov. 26, 1992).
MCP activity has been implicated in several disease states. For example, abnormally high expression of MCP in human leukemic cell lines has been reported. Kumatori, A. et al. PNAS 87:7071 (1990). Autoantibodies against MCP in patients with systemic lupus erythematosus ("SLE") have also been reported. Arribas, J. et al. J. Exp. Med. 173:423-427 (1990).
Agents which are capable of inhibiting the MCP complex are needed; such agents would provide a valuable tool for both those conducting research in the area of, for example, MCP activity, as well as those in the medical fields in order to, for example, control the deleterious effects of abnormal or aberrant MCP activity. The present invention is directed to these important ends.